Lean premixed combustion systems have been deployed on land based and marine fuel turbine engines to reduce emissions, such as NOx and CO. These systems have been successful and, in some cases, produce emission levels that are at the lower limits of measurement capabilities, approximately 1 to 3 parts per million (ppm) of NOx and CO, Although these systems are a great benefit from a standpoint of emission production, the operational envelope of the systems is substantially reduced when compared to more conventional combustion systems. As a consequence, the control of fuel conditions, distribution and injection into the combustion zones has become a critical operating parameter and requires frequent adjustment, when ambient atmospheric conditions, such as temperature, humidity and pressure, change. In addition to ambient condition changes, variation in the fuel's thermophysical properties will also change operational conditions leading to another source of variation that requires adjustment of the fuel turbine operational settings. The re-adjustment of the combustion fuel conditions, distribution and injection is termed tuning.
Controlled operation of a combustion system generally employs a manual setting of the operational control settings of a combustor to yield an average operational condition. These settings may be input through a controller, which as used herein shall refer to any device used to control the operation of a system. Examples include a Distributed Control System (DCS), a fuel turbine controller, a programmable logical controller (PLC), a stand-alone computer with communication to another controller and/or directly to a system.
These settings are satisfactory at the time of the setup, but conditions may change when tuning issues arise and cause an unacceptable operation in a matter of hours or days. Tuning issues are any situation whereby any operational parameters of a system are in excess of acceptable limits. Examples include emissions excursion outside of allowable limits, combustor dynamics excursion outside of allowable limits, or any other tuning event requiring adjustment of a turbine's operational control elements. Other approaches use a formula to predict emissions based on fuel turbine's operating settings and select a set point for fuel distribution and/or overall machine fuel/air ratio, without modifying other control elements, such as fuel temperature. These approaches do not allow for timely variation, do not take advantage of actual dynamics and emission data or do not modify fuel distribution, fuel temperature and/or other turbine operating parameters.
Another variable that impacts the lean premixed combustion system is fuel composition. Sufficient variation in fuel composition will cause a change in the heat release of the lean premixed combustion system. Such change may lead to emissions excursions, unstable combustion processes, or even blow out of the combustion system. Over the last twenty years, many economic and technological changes have occurred which have led to paradigm shifts in key operational inputs into fuel turbine combustion systems—namely fuel compositions requirements. One example of a fuel that is of considerable significance in this area is the use of liquefied natural gas (LNG).
LNG is becoming increasingly more prominent in the United States, Asia and South America. An inherent feature of LNG is variable gas composition as a “batch” of LNG is consumed. Since gas constituents with different volatilities (methane, ethane, propane, etc.) are vaporized at different rates (methane being one of the fastest to volatilize), methane concentrations typically continue to decrease as a “batch” of LNG is vaporized and subsequently consumed.
In addition, fuel producers are continually faced with economic and operational pressures to deliver “non-pipeline quality” fuel to their consumers. To this end, some suppliers have gone as far as to incentivize their customers to burn “off-spec” fuel by offering a reduction in the price per million BTU ($/MMBTU). As used herein, the concept of multiple-fuel burning combustion turbines will be discussed in terms of “pipeline quality” and “non-pipeline quality” fuels. However, it should be understood that while these are common terms to refer to a primary fuel source and a secondary fuel source or sources, they are intended to merely define first and second fuel sources, which may all be of pipeline quality or may not contain any pipeline quality fuel. In many cases, the “pipeline quality” fuel may be more expensive than “non-pipeline quality” but this is not required.
On marine based equipment each refueling of liquid fuel is an opportunity for a change in its physical properties depending on the source and grade of the fuel. Such changes frequently impact emission levels of the gas combustion turbines and may also impact the base load points of the propulsion or power plant.
These above criteria have caused increased pressure on gas turbine operators to operate their equipment using “non-pipeline quality” fuel or non-standard distillate. However, consumption of large quantities of this “off-spec” fuel may have detrimental effects on the combustion turbine system.
In addition, mis-operation of the combustion system manifests itself in augmented pressure pulsations or an increase in combustion dynamics (hereinafter, combustion dynamics may be indicated by the symbol “δP”). Pulsations can have sufficient force to destroy the combustion system and dramatically reduce the life of combustion hardware. Additionally, improper tuning of the combustion system can lead to emission excursions and violate emission permits. Therefore, a means to maintain the stability of the lean premixed combustion systems, on a regular or periodic basis, within the proper operating envelope, is of great value and interest to the industry. Additionally, a system that operates by utilizing near real-time data, taken from the turbine sensors, would have significant value to coordinate modulation of fuel composition fuel distribution, fuel or distillate inlet temperature and/or overall machine fuel/air ratio.
While real-time tuning of a combustion system can provide tremendous operational flexibility and protection for turbine hardware, a combustion system may concurrently experience a number of different operational issues. For example, most turbine operators of lean premixed combustion systems are concerned with exhaust emissions (NOx and CO) as well as combustor dynamics. It is not uncommon for both high NOx emissions and high combustor dynamics to coexist on a turbine. Additionally, tuning in response to one concern can make other constraints worse, for example tuning for low NOx can make combustor dynamics worse, tuning for high CO can make NOx worse, etc. It would be beneficial to provide a system whereby an algorithm is used to compare the current status of all tuning concerns, rank each concern in order of importance, determine the operational concern of most interest, and subsequently commence automated tuning to remediate this dominant operational concern.
Since many operators are incentivized to consume as much of the less expensive “non-pipeline quality” fuel as possible while mixing the non-pipeline quality fuel with pipeline quality natural fuel (and sending the resultant mixture to their fuel turbine combustion system), a means of real-time optimization of the ratio of non-pipeline quality to pipeline quality fuel is also desired.